In accordance with the practical application of optical communication systems, more highly advanced optical communication systems with large capacity and multi-functions have been required in these days. Functions such as the high speed generation of light signals, the change-over, the switching, etc. of light transmission paths are also required to be added to these systems. In a practical system, light signals are generated by directly modulating current injected into a semiconductor laser as a light emitting device. In accordance with this direct modulation, however, there are disadvantages in that high speed modulation more than several GHz is difficult to be obtained due to relaxation oscillation, etc., and that it is difficult to be applied to a coherent light transmission system due to the fluctuation of wavelength.
In order to overcome these disadvantages, a method in which an external modulator is used has been proposed. Especially, an optical modulator of waveguide type which is structured by waveguides fabricated in an electrooptic crystal substrate has advantages for the purpose in that size is small, efficiency is high, and response is fast.
On the other hand, an optical switch has been used for the purpose of changing-over light transmission paths, or switching light signals in a network system. The optical switch which has been practically used is of a structure in which a prism, a mirror, or an optical fiber, etc. is mechanically moved to switch light transmission paths. However, the mechanically operating optical switch has disadvantages in that operation speed is low, and size is large to result in the difficulty of realizing a matrix-patterned system.
In order to overcome these disadvantages, an optical switch of waveguide type using optical waveguides has been developed. This optical switch which is based on electrooptic effect has advantages in that response speed is high, a number of switch devices can be integrated, and reliability is high. Especially, an optical switch using a ferroelectric material such as a LiNbO.sub.3 crystal, etc. has characteristics that light absorption is small to provide low loss, efficiency is high for large electrooptic effect, and so on.
As apparatus of this kind, an optical control device such as an optical directional coupler type modulator or switch, a total reflection type optical switch, Mach Zehnder type optical modulator, etc. has been reported.
The high density integration of waveguide type optical switches has been researched and developed in these days, and has been described in "OQE88-147, the electronic information and communication engineering society" by Hiroshi Nishimoto, et al. In this report, a 8.times.8 matrix optical switch of waveguide type which is obtained by 64 switch devices integrated on a LiNbO.sub.3 substrate has been explained to provide the switching of light signals by the provision of 8 inputs and 8 outputs. On the other hand, an optical device composed of a single optical switch device such as an external optical modulator has been also researched and developed to improve its characteristics. Far such an optical switch device, parameters such as switching voltage (power), cross talk, light distinction ratio, loss, switching speed, stabilization of operation on surrounding atmosphere such as temperature, humidity, etc. are very important.
One type of a conventional optical switch comprises two waveguides having a portion of a directional coupler provided on a LiNbO.sub.3 substrate by thermal diffusion of Ti thereinto, and two control electrodes provided through a buffer layer on the portion of the directional coupler. The detail of the conventional optical switch will be explained in more detail later.
However, the conventional optical switch has a disadvantage in that an output level is fluctuated dependent on polarization of an input light signal. In other words, a power of an output light signal supplied from an output port is different between TE mode and TM mode, because a complete coupling length is different between the two modes. This is caused by the fluctuation, etc. among process batches, even if the directional coupler is fabricated to provide the same coupling length between TE and TM modes by controlling fabricating conditions.